Electronic Circuits ELCT604
Lecture 5: Analysis of BJT Amplifiers
Dr. Eman Azab
Assistant Professor
Office: C3.315
E-mail: eman.azab@guc.edu.eg
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
BJT Large Signal Analysis
 Voltage Amplifier using BJT
 Assume that we have instantaneous input voltage signal ‘vI’
𝑣O = VCC − 𝑖C R C
𝑣I
𝑖C = IS exp
VT
For Active Mode
ONLY
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
BJT Large Signal Analysis
 Voltage Amplifier using BJT
1.
𝑣BE = 𝑣I
Cutoff Mode
0 ≤ 𝑣I ≤ 0.5
2.
𝑣O = VCC
Active Mode
𝑣O > 𝑣CE,sat
0.5 ≤ 𝑣I ≤ 𝑣BE,sat
𝑣O = VCC − IS R C exp
3.
Saturation Mode
𝑣I ≥ 𝑣BE,sat
3
𝑣I
VT
𝑣O = 0.2
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
BJT as an Amplifier
 We can easily separate DC and AC Signals (Superposition)
 Under the assumption that the AC signal amplitude is very small, such that the
transistor’s mode will remain the same for the complete cycle
 BJT must work in Active Mode to avoid signal distortion
 DC Sources are used to make sure BJT operates in Active mode
 Input terminals of the amplifier are Base/Emitter
 Output terminals of the amplifier are Collector/Emitter
 Equivalent Circuit for Small Signal Analysis can be derived
𝑣BE = VBE + 𝑣𝑏𝑒
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
VBE + 𝑣𝑏𝑒
𝑖C = IS exp
VT
BJT Small Signal Model Derivation
 Assume BJT is in active mode & vbe<<VT
VBE + 𝑣𝑏𝑒
𝑖C = IS exp
VT
VBE
𝑣𝑏𝑒
𝑖C = IS exp
exp
VT
VT
𝑖C ≅ IC
𝑣𝑏𝑒
1+
VT
𝜕𝑖C
IC
𝑔m =
=
𝜕𝑣𝑏𝑒 VT
𝑖C = 𝑔m 𝑣𝑏𝑒 = 𝛽𝑖b
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
BJT Small Signal Model
 We can place a resistance between Base and Emitter to have a path
for the base current
𝑖C = 𝑔m 𝑣𝑏𝑒 = 𝛽𝑖b
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
𝑣𝑏𝑒
β
VT
𝑟π =
=
=
𝑖b
g m IB
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
BJT Small Signal Model
 Note that: Early effect can be taken into consideration
VA
𝑟o =
IC
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Analysis of BJT Amplifiers
 Objective: Calculate the voltage gain, Input and Output
Resistances
1.
2.
3.
4.
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Determine the DC operating Point (Deactivate AC signals)
Calculate the small signal model parameters: gm, rπ
Replace the BJT with its small signal model (DC sources are
deactivated)
Analyze the circuit to calculate the voltage gain, Input and Output
Resistances
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
BJT Amplifiers Configurations
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Saturday, February 20, 2016
Common Emitter Amplifier
 Objective: Calculate the voltage gain, Input and Output
Resistances
Input terminal Base
Output Terminal Collector
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Common Emitter Amplifier
1.
Calculate the DC Current
2.
Calculate gm and rπ
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
β
IC =
I≅I
1+β
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Common Emitter Amplifier
3.
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Draw the equivalent small signal model (Include ro if
given)
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Common Emitter Amplifier
Draw the equivalent small signal model (Include ro if
given)
4. Calculate the gain, input and output Resistance
3.
𝑣O
R B ∕∕ rπ
Av =
= −g m (ro ∕∕ R C ∕∕ R L )
𝑣sig
R B ∕∕ rπ + R sig
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
R in = R B ∕∕ rπ
R out = ro ∕∕ R C ∕∕ R L
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Common Emitter Amplifier
 Notes on Common Emitter Configuration:
Inverting Amplifier
 Gain is greater than unity
 High Input Resistance
 High Output Resistance

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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Common Base Amplifier
 Objective: Calculate the voltage gain, Input and Output
Resistances
Input terminal Emitter
Output Terminal Collector
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Common Base Amplifier
 Voltage gain, input and Output Resistance (ro is neglected)
𝑣O
g m (R C ∕∕ R L )
Av =
=
RS
𝑣sig 1 +
r
(R E ∕∕ π )
1+β
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
R in
rπ
= R S + (R E ∕∕
)
1+β
R out = R C ∕∕ R L
Common Base Amplifier
 Notes on Common Base Configuration:
Non-Inverting Amplifier
 Gain is greater than unity
 Low Input Resistance
 High Output Resistance

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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Common Collector Amplifier
 Objective: Calculate the voltage gain, Input and Output
Resistances
Input terminal Base
Output Terminal Emitter
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Common Collector Amplifier
 Voltage gain, input and Output Resistance
𝑣O
Av =
=
𝑣sig
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1 + β (ro ∕∕ R L )
rπ + 1 + β (ro ∕∕ R L )
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
R sig
1+
+ R sig
RB
Common Collector Amplifier
 Voltage gain, input and Output Resistance
R in = R B ∕∕ rπ + 1 + β ro ∕∕ R L
R out
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rπ + R B ∕∕ R sig
= ro ∕∕
1+β
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Common Collector Amplifier
 Notes on Common Collector Configuration:





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Non-Inverting Amplifier
Gain is less than unity
Emitter Follower (Buffer)
High Input Resistance
Low Output Resistance
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Common Emitter with emitter Resistance
 Exercise:
 Find theVoltage gain, input and Output Resistance
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.